<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        * {
            margin: 0px;
            padding: 0px;
        }
        .box{
            height: 500px;
        }
        .left, .right, .center{
            float: left;
        }
        .left, .right{
            width: 300px;
            height: 300px;
        }
        .center{
            width: 100%;
            background-color: green;
        }
        .left{
            background-color: red;
            margin-left: -100%;
        }
        .right{
            background-color: blue;
            margin-left: -300px;
        }
        .inner{
            margin-left: 300px;
            margin-right: 300px;
            background-color: pink;
            height: 300px;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="center">
            <div class="inner">中</div>
        </div>
        <div class="left">左</div>
        <div class="right">右</div>
    </div>
</body>
</html>